1. Field of the Invention
This invention relates to a method and apparatus for investigating the mechanical properties of a material. In particular, the present invention relates to the noninvasive use of ultrasound for assessment of bone mass and strength by analyzing the reflection of ultrasound waves at the soft tissue-bone interface.
2. Outline of Problem
The need for assessing the mechanical properties of a material is found in a wide variety of applications. For example, it is necessary to test the strength of materials in a wide variety of industrial applications. In many applications, destructive testing techniques can be utilized to determine the mechanical properties of a material. In other types of material testing, the material is easily accessible for analysis and nondestructive techniques can be utilized. Further, many materials are homogenous on a macroscopic level, permitting simplified techniques based upon assumptions of the homogeneity.
While many different techniques have been developed for investigating the mechanical properties of materials, many types of materials are not accurately analyzed using conventional techniques. For example, in many medical applications, it is desirable to determine the mechanical properties of the material for proper diagnosis and treatment. In many medical applications, however, the testing is difficult to analyze because destructive testing cannot be used, invasive types of testing are undesirable, and the non-homogenous nature of biological tissue presents unique problems. A good example is the need for accurate measurement of bone mass and bone strength as an indication of resistance to fracture.
The capability to accurately assess resistance to fracture would have great clinical significance in the diagnosis and treatment of numerous medical problems such as osteoporosis. In osteoporosis, bone mass is lost gradually and progressively thus decreasing the mechanical strength of the skeleton until even minimal trauma results in bone fracture. Osteoporosis affects one in three women and one in five men over the age of 60. Over 80% of the one million fractures sustained yearly by women over the age of 50 in the United States is a consequence of osteoporosis. Half of the patients with fractures resulting from osteoporosis never recover normal functions, and 30% progress to premature death, 10% dying within three months because of peri- and post-operative complications. However, treatments exist which alter, delay or reverse the progression of osteoporosis if the disease is accurately diagnosed before fracture occurs. The development of improved treatments would be greatly facilitated by a technique capable of delineating their effectiveness.
Unfortunately, assessment of osteoporosis is difficult. Only a small portion of elderly osteoporotic women and men have whole skeletons demonstrating a discernible degree of osteoporosis. That is, different sites in the skeleton are associated with different degrees of osteoporosis. Further, bones are inherently non-homogeneous making assessment difficult even on a localized basis. Finally, the nature of the disease dictates an accurate, non-invasive technique for diagnosis and assessment.
3. Description of Current Methods
Currently there is no widely accepted accurate method to diagnose and assess bone strength as an indication of resistance to fracture. Indeed, osteoporosis is usually diagnosed only after a fracture occurs. A number of methods have been proposed, however, all of which have a number of problems. Biochemical analysis of bone tissue correlates very poorly with bone strength, because osteoporosis is the result of long term metabolic deficiencies and strong temporal correlation between the disease and biochemical analysis is not clear. Invasive methods such as bone biopsies are usually accurate determinations of bone mechanical properties, but are only accurate at the site of a biopsy. That is, the bone biopsy taken from the region of the iliac crest may give little indication of the extent of osteoporosis of the lumbar vertebrae or femoral neck.
Additionally, there are a number of non-invasive techniques which have been introduced to diagnose and assess the extent of osteoporosis. For example, radiogrammetry has been used to measure the thickness of the cortex, photodensitometry measures the photographic density, and single-photon absorptiometry measures mineral content. While these methods are useful in measuring bone density and bone mineral density in the appendicular skeleton, they are of little value in assessing osteoporosis in the spine or hip. Dual-photon and computed tomography are usable in the spine or hip, but are of limited value in other respects. In dual-photon absorptiometry, only an integrated attenuation is measured, thus cortical and trabecular bone are not independently assessed. Calcification outside the bone of interest, bone shape, and vertebrae compression and deformity can alter the results. Computed tomography (single energy) is biased by marrow fat concentration, but can measure geometric non-homogeneities and in particular, differentiate between cancellous and cortical bone. However, neither dual-photon nor computed tomography can accurately predict the tendency of bone to fracture.
Neutron activation analyses can quantify the presence of calcium in the whole body or at selected sites. The doses involved are typically in the range of 0.3 to 3 rem. However, neutron activation analysis cannot accurately predict the tendency of a bone to fracture and appears to be too expensive for practical use.
Nuclear medicine studies utilize radionuclides having particular skeletal bone affinity and are good indicators of bone turn over in kinetic parameters. Unfortunately, nuclear medicine studies are difficult and must be carried out at frequent intervals for long time periods and nevertheless do not yield a good indication of the tendency of bone fracture.
Several techniques have been proposed to study the mechanical properties of bone directly in vivo. For example, a variety of instruments have been designed which use static or frequency loading of the bone to measure the resonant frequency and impedance as well as measurement of velocity and elastic modulus by ultrasound. U.S. Pat. Nos. 4,361,154 and 4,421,119, Pratt, Jr. are indicative of past uses of ultrasound to analyze bone strength. In such preexisting ultrasound techniques, an ultrasound pulse is launched from the transducer on one side of the bone and received at a transducer on the other side of the bone. The distance between transducers is used to determine the effective velocity of the pulse through the bone and tissue, and the assumed speed of the ultrasound through the soft tissue subtracted to give an apparent velocity of the ultrasound through the bone. However, such an ultrasound technique as described in the Pratt, Jr. patents is not an accurate prediction of the tendency of fracture for a number of reasons.
First, the velocity through the bone is measured indirectly and hence inaccurately, because the technique measures only the time difference and the path of travel is measured by a non-ultrasound method. Second, the measurement of the path of travel is inaccurate because bone is non-homogenous and anisotropic--meaning that the path of travel of the ultrasound is not equal to the distance between the transducers, which is the distance measured. Third, even assuming the time and distance of the path of travel could be accurately measured, the result would yield only a pressure wave velocity through the bone, limiting its usefulness. Because bone is mineralized tissue, ultrasound waves propagating through it are not merely pressure waves as in soft tissue, but have a significant transverse or shear component which must be assessed to adequately investigate the mechanical properties of bone. Finally, the ultrasound signal is strongly attenuated by bone at frequencies of 1 megaHertz, making only low frequency ultrasound usable. Thus, present ultrasound techniques for investigating the mechanical properties of materials such as bone, present inherent difficulties in accurately predicting mechanical properties, such as the likelihood of a bone fracture.